The issue of climate and global warming is a hot topic nowadays. A Norwegian governmental report discussed aspects of the electrification of new and existing devices on the Norwegian continental shelf with power from the mainland that would reduce the green house gas emission form the petroleum sector. One type of offshore oil and gas installations that need to be electrificated are so called Floating Production Storage and Offloading (FPSO). The FPSO's can be divided in two main groups by their strategy for seaworthiness:                Ship shaped FPSO with a turret in order to enable mooring independent weather vaning.        Non weather vaning FPSO concepts that have a permanent mooring dependent heading.        
A weather vaning FPSO is a vessel comprising a hull and a stationary turret. Today, the turret often has slip ring systems inside for transmission of electrical power. These electrical slip rings are usually air or oil insulated. One of the largest known slip ring based swivel in operation have an maximum power in the 40 MVA range, but they all have disadvantages of being subject to friction, wear, intermittent contact, and they have limitations on the currents and voltage levels that can be accommodated with the available space versus risk for electrical or mechanical damage.
Swivel technology exists today that can handle lower voltage levels and for relatively short distances, for example 10-30 km. The typical electrical power exchange limit for a ship shaped weather vaning FPSO designed for offshore co-generation has been identified to be in the 30 kV and 30 MW range, which often is a bit too low for several offshore installations today. These are some of the limitations that make it difficult to provide electrification of the FPSO's.
Today, the weather vaning FPSO (typically ship shaped) could generally have a power from shore or “next” neighbour power interface through the turret via a swivel arrangement. This is feasible for an up to 30 MW+ power export/import design with the 2007 technology status. If this system is operated around the 10-20 MW import level for most of the expected project life time it can bridge quite long distances, i.e. local offshore power generation corresponding with the offshore process heat demand. No local cogeneration, i.e. total electrification, with 36 kV power from a remote source could give cases with voltage instability.
For power transmission higher than 30 MW there are some limitations that can be defined as technology qualification needs mostly related to the electric ac-power swivel system design and further technology development, exceeding the typical IEC classification 18/30 (36) kV. For this, an electrical swivel design based on an EExp approach or similar, appear to be preferable.
The power and voltage level limits of an electrical power swivel on the turret can be avoided by installing a rotary transformer with the turret instead. The rotary transformer is a well known technique that has been present for many years, and has many different applications. As known by a person skilled in the art, a rotary transformer is utilized to couple electrical signals between two parts which rotate in relation to each other. Some examples of applications of rotary transformers are air-bag systems in cars, videocassette recorders (VCR), different types of electrical machines, spacecraft applications etc.
A similar problem is present in relation to wind turbines where the turbine as adapted to head toward the wind and thus be able to rotate relative to the vertical axis. Wind turbines can be located on shore or off shore. The choice of location is based on several parameters and a good location has a wide, open view and few obstacles. Most wind turbines have been placed on shore, but off shore locations have become more and more interesting. The winds off shore is generally less turbulent than on shore, and therefore are off shore wind turbines expected to have longer lifetimes than on shore wind turbines. As the world becomes more and more populated the lack of space is a problem regarding wind turbine locations. Thus, off shore locations are becoming even more preferable. An example of a off shore wind turbine park is the Horns Rev wind farm located 14 km from the west cost of the Danish Jutland, where the wind turbines are positioned in the sea ground. This wind farm has wind turbines of the type V80-2,0 MW from the Danish wind turbine production company Vestas. Vestas has also a mega Watt wind turbine called V90-3.0 MW, which was introduced in 2002. However, due to the increasing energy consumption, there is a future need for wind turbines producing even more power.
The existing types of wind turbines, such as for example the Vestas V80-2,0 MW wind turbine, have a high voltage transformer located in the back of the nacelle, which can add to a stability problem due to the large weight of the transformer placed in the top of a relatively high tower. Even though there has been a great reduction in weight of wind turbines during the last time period by for example using new lightweight materials, there is still a need for reduction of the weight of for example the nacelle. Off shore wind turbine parks can be positioned in the sea ground or on floating constructions. Wind turbines being floating, have several advantages relating to for example utilisation of the wind power compared to the non-floating wind turbines.
Today, wind turbines use a rotary unit for conversion of power and signals from the rotating wind turbine blades. Such a system for conversion of electrical power can be a slip ring system. A person skilled in the art will know that a slip ring system is an electromagnetic device allowing conversion of power from a stationary to a rotating structure or vice versa. One type of slip rings used in wind turbines are for example slip rings with fibre brush technology. Slip rings have disadvantages of being subject to friction, wear, intermittent contact, and they have limitations on the currents and voltage levels that can be accommodated with the available space versus risk for electrical or mechanical damage. So the increasing demand for power is difficult to provide with today's wind turbine technology.
The power and voltage level limits of today's wind mills can be exceeded by installing a three-phase rotary transformer with the wind turbine instead of for example the slip ring system. A rotary transformer is a well known technique that has been present for many years, and has many different applications. As known by a person skilled in the art, a rotary transformer is utilized to couple electrical signals between two parts which rotate in relation to each other. Some examples of applications of rotary transformers are air-bag systems in cars, videocassette recorders (VCR), different types of electrical machines, spacecraft applications etc.
The patent publication EP 1 742 235 A2 discloses an electrical power generator, such as a wind turbine comprising a rotary transformer. The rotary transformer transforms the power from the rotating frame of the generator to the stationary frame. In a multiphase power system there will be one rotary transformer for each phase. The transformers arranged coaxially in sequence and packaged as a single assembly. Each transformer has then two legs, so a three-phase system with three rotatary transformers will have six legs. The transformer core of the generator is constituted by soft iron. As known for a person skilled in the art, a soft iron transformer core is normally used for high frequencies, and not for power conversion due to, for instance, large losses and high temperature at high powers.
Norwegian Patent NO 165220 describes a rotating transformer based on a complicated system relying on load dependent torque, and will not be suitable for high power transmission systems.
Therefore, it is a need for technology development that provides solutions which reduces the green house gas emission by using alternative energy sources, and at the same time satisfies the world's increasing power demand and consumption. The new technology must exceed the existing limits of today's wind turbines regarding power and voltage range, and also avoid the use of slip rings. As wind turbines, driven by higher power ratings, grow bigger in magnitude, the weight of components placed ever higher above ground becomes critical. Solutions that, in a reliable manner, move mass downwards within the wind turbine are crucial in the development of larger wind turbines.